The purpose of this study to examine the safety and efficacy of OPC-6535 and determine its optimal dose by once-daily oral administration at 0, 25, or 50 mg for 8 weeks in combination with a fixed oral dose of 5-aminosalicylic acid (5-ASA) or in combination with a fixed oral dose of 5-ASA and enteral nutrition in patients with active Crohn's disease.